The present invention relates to a scanning type charged particle beam apparatus for obtaining a scanning image of a specimen by detecting a signal generated from the specimen by irradiation of a primary charged particle beam.
As well known in the art, in a charged particle beam apparatus having an electron gun comprised of, for example, a cathode, an extraction electrode and an accelerating electrode, when a condition for the electron gun as principally represented by extraction voltage V1 applied across the cathode and the extraction electrode or accelerating voltage V0 applied across the cathode and the accelerating electrode is changed, the position of a virtual cathode changes. This is accounted for by the fact that the intensity of an electric field at the tip of cathode changes to change the trajectory of a charged particle beam emitted from the surface of cathode and besides its trajectory at the extraction electrode, which trajectory coincides with a straight line extending from the virtual cathode existing inside the cathode, is also changed. Therefore, when the extraction voltage V1 and accelerating voltage V0 are made to be variable, an optical condition must be set by taking a change in position of the virtual cathode into consideration. Conventionally, as described in JP-A-11-176367, values of position change amounts calculated in advance through simulation are stored and used for controlling the optical condition.